Certain sea slugs steal chloroplasts from the algae they eat and use them to photosynthesize, essentially running on solar power like a plant. The most well-studied species, Elysia chlorotica, can survive on nothing but light for up to 10 months thanks to these stolen chloroplasts. It’s one of the most remarkable examples of one organism hijacking another’s cellular machinery for its own benefit.
How Sea Slugs Steal Chloroplasts
The process is called kleptoplasty, and it works like a very selective form of digestion. When a sea slug feeds on algae, it pierces the algal cells and sucks out their contents. The slug’s digestive system breaks down almost everything: the cell walls, the nucleus, the other organelles. But the chloroplasts are spared. Instead of being digested, they’re absorbed intact by cells lining the slug’s digestive gland, a network of tiny tubes (called digestive tubules) that branch throughout the slug’s body.
Once inside those cells, the chloroplasts remain structurally intact and, in many species, fully functional. They continue doing what chloroplasts do: capturing light energy and converting carbon dioxide into organic carbon through photosynthesis. The slug’s body is flat and leaf-shaped, which maximizes the surface area exposed to light, essentially turning the animal into a living solar panel.
Energy From Sunlight
The chloroplasts provide real, measurable nutrition. In Elysia chlorotica, the captured chloroplasts serve as the sole energy source for the animal for over 10 months, which covers the slug’s entire adult lifespan. Imaging studies using isotope tracers have confirmed that the chloroplast-bearing digestive tubules actively incorporate inorganic carbon in the light, meaning they’re genuinely fixing carbon dioxide into usable organic molecules inside the slug’s tissues.
Not every species benefits equally. Some sea slugs retain functional chloroplasts for only a few weeks, while others maintain them for months. Four species within the genus Elysia are considered the champions of long-term chloroplast retention: E. chlorotica, E. viridis, E. timida, and E. crispata. Elysia viridis, for instance, keeps its chloroplasts photosynthetically active for up to 12 weeks. By contrast, a closely related slug called Placida dendritica eats the same algae but retains chloroplasts that no longer function.
A Genetic Puzzle
Here’s what makes this especially strange: chloroplasts can’t operate alone. In a plant or algal cell, more than 90% of the proteins a chloroplast needs are encoded not in the chloroplast’s own DNA but in the cell’s nucleus. When the slug digests the algal nucleus, those genes should be lost, and photosynthesis should grind to a halt within days. Yet in some species, it continues for months.
Research on Elysia chlorotica revealed a surprising explanation. At least some of the genes needed to support chloroplast function have been transferred from the alga’s nuclear DNA into the slug’s own genome. This is horizontal gene transfer, the movement of genetic material between unrelated organisms rather than from parent to offspring. One key gene, psbO, which encodes a protein essential for the water-splitting step of photosynthesis, was found to be identical in the slug and its algal prey, Vaucheria litorea. The gene has integrated into the slug’s germline, meaning it gets passed from one generation to the next. Additional genes involved in light-harvesting have also been identified in the slug’s DNA, with sequences nearly identical to those of the alga.
This means the slug isn’t just borrowing chloroplasts temporarily. Its own genome has been permanently altered by its relationship with algae, allowing it to produce some of the proteins the stolen chloroplasts need to keep working.
Oxygen Inside the Body
Photosynthesis produces oxygen as a byproduct, and because the chloroplasts are spread throughout the slug’s branching digestive system, that oxygen is generated inside the animal’s tissues. Research on Elysia viridis found that slugs exposed to brighter light showed improved heat tolerance, likely because internal oxygen production helped offset the higher oxygen demands that come with rising temperatures.
That said, the internal oxygen supply doesn’t cover all of the slug’s needs. Measurements consistently show that these slugs still take in oxygen from the surrounding water, meaning photosynthesis supplements their oxygen supply rather than replacing it entirely.
Built-In Camouflage
The chloroplasts also change the slug’s appearance. Without them, Elysia chlorotica is brown. With a full load of chloroplasts distributed through its body, it turns bright green. Combined with its flat, leaf-like shape and wing-like body flaps called parapodia, the slug closely resembles the aquatic plants and algae in its habitat. This coloration functions as camouflage, helping the slug blend in and avoid predators.
How Much Do Chloroplasts Really Matter for Survival?
This is where the science gets more complicated. While some species clearly photosynthesize and fix carbon, experiments testing whether photosynthesis is strictly necessary for survival have produced surprising results. Researchers kept Elysia timida in complete darkness for nearly three months and found they survived just as well as slugs kept in the light. In another experiment, slugs treated with a chemical that blocks photosynthesis lost weight at exactly the same rate as untreated controls over 55 days. In both cases, survival and weight loss were indistinguishable between groups.
These findings suggest that for at least some species, the chloroplasts may not be providing as much usable energy as once assumed, or the slugs may have other metabolic strategies (like living off stored lipids or relying on their microbiome) that sustain them during starvation. The picture varies by species. Elysia chlorotica appears to depend heavily on its chloroplasts across its full 10-month lifespan, while others like E. timida and Plakobranchus species seem more resilient without active photosynthesis.
Which Slugs Do This and What They Eat
Kleptoplasty occurs in a group of sea slugs called sacoglossans, but only a handful of species retain chloroplasts long enough for them to be functionally useful. The best-studied species and their preferred algae include:
- Elysia chlorotica: feeds exclusively on Vaucheria litorea, a yellow-green alga found in shallow estuaries along the eastern coast of North America.
- Elysia timida: relies on a single alga, Acetabularia acetabulum, a Mediterranean species sometimes called the mermaid’s wineglass.
- Elysia crispata: a more flexible eater found in the Caribbean, drawing chloroplasts from a wide variety of algae including Bryopsis, Halimeda, and Penicillus species. Juveniles start with thin, filamentous algae that are easier for their small mouthparts to pierce.
- Elysia viridis: found in European waters, retaining functional chloroplasts for up to 12 weeks from various green algae.
The range of algal sources matters because different chloroplasts appear to function differently inside the slug. Elysia crispata fed on Bryopsis versus Acetabularia, for example, acquires distinct populations of chloroplasts, and the longevity and function of those chloroplasts can vary depending on their origin.